This invention deals with a remoistenable adhesive composition which is especially suitable on reinforced gummed tapes. Such adhesives should exhibit characteristics of high initial tack and rapid bond formation which are essential for high speed packaging operations, for example, in carrying out automatic or semiautomatic processes. Furthermore, the adhesive composition should exhibit rheological properties which are appropriate for pregumming lightweight kraft via transfer roll coating methods. Pregumming, a step for the application of a remoistenable adhesive to a primary web prior to lamination, is the type of coating preferred by the gummed tape industry. The web strength of a single ply of kraft paper is considerably less than that of a laminated web. Unless the rheology of an adhesive permits it to transfer evenly and uniformly from the coating roll to the web, numerous web breaks are encountered during the pregumming step. Postgumming, a step for the application of the adhesive following lamination, often results in the deposition of a nonuniform coating due to irregularities in the caliper of the laminated, fiberglass-reinforced web with an associated reduction of adhesive properties in certain areas.
In general, the gummed tape industry has chosen animal glue adhesives for use on reinforced sealing tape. Availability, low price and good adhesion properties coupled with ease of compounding and machining have made animal glues the preferred raw material. However, the instability of domestic production and fluctuations in price have made it necessary to look for alternative adhesive compositions.
Low amylose starches, such as those derived from the waxy maize hybrid variety of corn, are commonly used in remoistenable adhesives for nonreinforced kraft paper tape. By themselves, however, gelatinized waxy starches lack the cohesive strength and high initial tack required for a reinforced sealing tape adhesive.
One approach suggested for improving the adhesion properties of waxy, starch-based compositions is through the addition of tack-inducing polymers or copolymers. For example, the U.S. Pat. No. 3,696,065 of Hoffman et al. (1972) describes an admixture of low amylose starch and a copolymer of vinyl acetate and vinyl pyrrolidone. A blend of waxy starch and poly-(vinyl methyl ether-maleic acid monoalkylester) is described in the U.S. Pat. No. 3,988,495, of Lowey and Frommherz (1976). An adhesive composed of polyacrylamide and waxy starch is the subject of the recently issued U.S. Pat. No. 4,105,824, of Monte (1978).
While such polymers and numerous other tack-increasing compounds do impart improvements to the starch-based compositions, admixes of the type described in the above-noted patents have not gained wide acceptance for use on reinforced tapes because of their inability to match the adhesion characteristics of animal glues at viscosities suitable for transfer roll coating. The composition of polymer/copolymer in the adhesives is usually strictly limited due to solubility, viscosity and other rheological requirements. High proportions of added polymer can dramatically and unfavorably increase the viscosities of these adhesive compositions, necessitating a reduction in solids in order to maintain machinable viscosities, e.g. when spreading and leveling from transfer rolls. The slower operating speeds and the higher oven temperatures required to dry such low solids adhesives result in substantially higher costs.
The molecular weight of polymer which can be used in the aforementioned admixtures is also limited if the adhesive is to be applied via transfer roll coating methods. Transfer roll coating is a general term for coating methods that apply a predetermined or pre-metered amount of adhesive from a coating roll onto the surface of a web. Those skilled in the art of paper coating recognize that there are many variations of the above-described roll application of adhesives, but that all are based upon similar principles. Reverse roll, gravure, kiss roll, and offset coaters are four commonly used transfer roll coaters. One generally avoids the use of adhesives in the form of high molecular weight, viscoelastic, cohesive admixture because, at best, they are extremely difficult to apply from transfer roll coaters.
Starches which have been chemically modified in the granular state are commercially available. The elaborate procedures required in these reactions and the expensive spray drying employed usually demand high prices for these modified products. On the other hand, disadvantages are encountered in reacting ungelatinized starch molecules with other additives, including a low degree of conversion in the reaction and an alteration of the starch gel and its resulting rheological properties.
Because starch must be gelatinized by the gummed tape producer prior to the application of any starch-based coating to a substrate, it would be most convenient for the converter to simultaneously carry out the gelatinization process with a graft of vinyl monomers to the starch via free radical initiation of the graft copolymerization reaction. For this type of combined reaction to be feasible in a batch process on a production basis, the following requirements must be met:
1. Reaction time should be short.
2. Reaction must be able to proceed at high solids.
3. The reaction mechanism must favor grafting, keeping the homopolymerization of the vinyl monomer to a minimum.
4. The product must be substantially free from unreacted monomer when it leaves the cooking vessel.
5. The product must be stable with holding time.
It is obvious that the reaction must also be reproducible and yield products which have the desired adhesion and rheological characteristics.
With modern coating equipment, the application of 20 pounds per 3000 ft..sup.2 of adhesive on a 72" width web at a rate of 800 feet per minute is within the normal scope of operating conditions. Unless cooking vessels of substantial capacity are used, the reaction time must be short enough to meet production demands. In the event a batch goes awry and cannot be used in production, a rapid reaction also reduces the recovery time.
The reaction must be able to proceed at a minimum of 40% and preferably at least 45% by weight of solids, with reference to water as the liquid carrier, if the tape manufacturer is to economically coat the freshly prepared adhesive composition. While many polymer reactions run well in very dilute solutions, the same is not true where the reaction liquid has a high solids content. The free movement of molecules in the reaction may be hindered in the presence of a high proportion of solids, thereby resulting in the formation of undesirable by-products such as homopolymers.
The mechanism or conditions of the reaction must favor graft copolymerization if the adhesive composition is to be applied via transfer roll coating methods. However, ethylenically unsaturated compounds are extremely unstable to free radical initiation, polymerizing in the presence of free radicals by a chain reaction mechanism such that at any given time only the monomer and its high molecular weight polymer exist in the reaction solution. The in situ homopolymerization of a vinyl monomer produces the same adverse effects on the rheological properties of a coating as does the addition of a high molecular weight polymer to starch as previously noted. The homopolymer cannot be economically separated from the starch-based adhesive composition, and excessive homopolymerization renders the entire batch unusable due to the poor viscoelastic behavior of the product. It is also desirable that complete reaction of the monomer take place during the cooking procedure. Federal regulations severely limit the amounts of free monomers which are permissible in adhesives which may come into contact with foods. The toxicity of many vinyl monomers is recognized, and the exposure of workers to this hazard is minimized by ensuring a complete reaction of monomers before leaving the cooking vessel.
The copolymer composition should also be stable over an extended holding time. In the event that a coating line goes down, it is important for the adhesive to retain its optimum adhesive properties over a 6- to 8-hour holding period.
The U.S. Pat. No. 3,770,672, of Yoshizawa et al. (1973) describes a process for producing an adhesive composition containing a copolymer of enzymatically decomposed starch and vinyl monomers. The initial step in this earlier disclosed process is the decomposition of the starch via alpha-amylose. Vinyl monomers are then copolymerized with the enzymatically reduced starch molecules. Batch cycle time for the described process ranges from 3 to 10 hours. Viscosities of the compositions produced in accordance with this patent vary from 6,000 to 12,000 cps as measured by a rotary viscometer at 10 r.p.m. and 55.degree. C. Such compositions can be applied to kraft webs by extrusion coating methods, but their high viscosities make them unsuitable for transfer roll coating applications.
The U.S. Pat. No. 3,640,925, of Touzinsky et al. (1972) discloses processes for simultaneously gelatinizing starch and grafting ethylenically unsaturated monomers to the starch. The patentees claim success in a free radical initiated grafting reaction with a large number of polymerizable monomers. It is further claimed that the graft copolymerization can be carried out with starch slurries having a solids content of up to 43% by weight, the preferred starch solids content being about 1 to 35% by weight. It has been found in making the present invention, however, that in compositions containing over 40% total solids, e.g. about 45-55% by weight as the solids range most frequently required for adhesive compositions used by gummed tape manufactures, a homopolymerization of the ethylenically unsaturated monomers strongly competes with the grafting reaction and causes an undesirably high viscosity in the final product. This problem has not been previously noted and there is no readily predictable solution for this kind of problem.